The present invention relates generally to an apparatus and method for elimination of abnormal heart rhythms or arrhythmias. More particularly, the present invention relates to an ultrasonic catheter and method for delivering ultrasonic energy to the heart for selectively ablating cardiac tissue to restore normal heart rhythms.
Currently there are a number of medical and surgical treatments for cardiac arrhythmias. Medical treatments are principally through the use of antiarrhythmic drugs which slow intra-cardiac impulses conduction or refractoriness which sustains an arrhythmia once started. All antiarrhythmic drugs have undesirable side effects. For example, nausea, vomiting or diarrhea occur in about 40-60% of patients treated with quinidine. Lupus, an immunoreactive syndrome characterized by high antinuclear titers in the blood, diffuse arthralgia, pleural and pericardial effusion occur in about 30% of the patients taking procainamide for longer than six months. Only recently have the proarrhythmic effects of these drugs begun to be fully appreciated. For example, in a recent National Institutes of Health sponsored study it was found that post-myocardial infarction patients who were treated with two of three antiarrhythmic drugs had a threefold higher sudden death mortality than those given placebo.
Surgical treatments offer a second therapeutic option in the treatment of cardiac arrhythmias. Surgical methods permit localization of the origin of the arrhythmia or a critical part of the electrical conduction circuit during open heart surgery. When accessed in this manner, the arrhythmia may be eliminated by excising myocardial tissue or ablating the tissue using cryothermia or laser. For example, some patients are born with an anomalous connection between the atrium and ventricle known as Wolff Parkinson White Syndrome. These anomalous conduction pathways can be surgically cut during open heart surgery.
Surgical treatment of arrhythmias has an associated mortality of less than 1% in treating patients with Wolff Parkinson White Syndrome and morbidity is not significant. However, surgical treatment of patients with ventricular arrhythmias has an associated 10% operative mortality. Open heart surgery for the treatment of cardiac arrhythmias is clearly not a desirable therapeutic modality.
Devices, commonly known as "pacemakers", are medical devices which have become widely used in the treatment of ventricular cardiac arrhythmias. These devices consist of programmable implanted units that either stimulate cardiac contractions by a train of electrical impulses or depolarize the heart to stop the arrhythmia, at which time normal sinus rhythm resumes. The devices which depolarize the heart are known as automatic, implantable cardioverter defibrillators (AICD) and have become accepted for treatment of ventricular arrhythmias which do not respond to drug treatment. Implanting AICD devices requires open chest surgery with the total cost of the device and implantation ranging from $35-50,000. Infection which requires removal of the device occurs in 2-4% of the cases and operative mortality ranges from 1-4%.
Myocardial tissue ablation is another therapeutic modality for treatment of arrhythmias. Tissue ablation techniques generally use an energy source to transmit either electrical or thermal energy to selected myocardial tissue to cause an ablative effect.
Current tissue ablation techniques include use of one of i) direct current; ii) radio frequency energy; iii) microwave energy; iv) cryothermia; or v) laser energy. In 1982 two separate investigators introduced the use of catheters to deliver a direct current electrical charge to myocardial tissue. Endocardial catheters were inserted percutaneously to the atrial-ventricular (AV) node region. The procedure attempted to totally eliminate electrical conduction between the atrium and the ventricle and is performed to treat atrial fibrillation or other arrhythmias involving rapid conduction of electrical impulses around or through the AV node. Subsequently, catheter-based delivery of direct current energy was extended to treat anomalous pathways as well as ventricular arrhythmias.
The use of direct current energy entails the endocardial generation of several hundred joules through application of about 2,000-4,000 volts of electricity for a few milliseconds. Tissue damage due to direct current shock may occur due to thermal injury, barotrauma or the induction of an electrical field in the tissue. A principal disadvantage associated with use of direct current energy is the difficulty of controlling the application of energy. The direct current myocardial tissue ablation techniques must be performed under general anesthesia due to the painful muscular contractions associated with application of the direct current energy. Complications include the danger of inducing ventricular tachycardia in 5% of the patients or perforation of the heart, tamponade, hypotension, shock and cardiac embolization, which are noted in about 15% of the patients. Use of direct current energy has also been known to damage the catheters used to deliver the voltages. Catheters used for application of direct current energy for myocardial tissue ablation are usually diagnostic electrophysiological catheters which are not made to withstand the applied voltages. As a consequence, the damaged catheter may generate an electrical discharge at a non-intended location.
In 1986 the use of radio frequency energy for cardiac ablation was introduced. This method has met with widespread acceptance and success in treating supraventricular arrhythmias. As result, radio frequency energy has become the dominant energy source used for myocardial tissue ablation. Catheter-based delivery of radio frequency energy causes thermal tissue damage as a result of the electrical current flow to the tissue. Radio frequency energy uses sinusoidal electrical current, in the range of 40-60 volts, directly applied to the tissue. Limitations on the use of radio frequency include low energy generation which limits the size of the ablated area, the resulting need for precise intracardiac localization, the formation of blood clots on the electrode once the electrode reaches 90.degree.-100.degree. C. and the decrease of power delivered to the tissue as the energy source moves away from the tissue. The latter factor is, perhaps, the most limiting. Since power delivered to the tissue decreases to the fourth power from the point of delivery from the catheter, the depth of tissue penetration is limited. This renders the radio frequency techniques unsuitable for certain arrhythmias, especially, those originating in the left ventricle. Additionally, there has been no mapping technique developed for use with the radio frequency catheters which permit rapid and precise localization of the energy source relative to the myocardium.
Microwave energy is under investigation as an energy source for cardiac tissue ablation. However, many of the practical limitations associated with radio frequency energy apply to microwave energy. As with radio frequency energy, power delivered by microwave energy decreases exponentially from the point of delivery, therefore tissue penetration may be limited, albeit to a lesser degree than with radio frequency energy. Additionally, because of its relatively long wavelength at the frequencies under investigation, microwave energy is extremely difficult to focus.
Cryoprobes, cooled to -70.degree. C., are commonly used to ablate cardiac tissue during open heart surgery. However, to deliver this degree of cooling to the tip of the catheter, the catheter has to be so large in diameter (11-12 French), that perforation of the cardiac tissue is a danger.
Finally, laser energy delivered through a pervenous catheter has been used to successfully ablate the AV node in canine experiments. Despite this success, there remains a serious concern relating to heart perforation, optical fiber tip deterioration, fragility of the optical fiber, and the lack of optimal portable instrumentation for laser energy generation, monitoring and cardiac mapping.
While the use of catheter-based energy delivery systems in ablation of myocardial tissue are clearly known, each of the systems known, used or under investigation suffer from one or more of the above mentioned shortcomings. The present invention has been developed to provide an alternative method and apparatus for intra-cardiac ablation of myocardial tissue to eliminate cardiac arrhythmias.